A transistor such as a field-effect transistor (FET) should preferably be excellent in off-state leakage characteristics, i.e., have an extremely low off-leakage current. Recently, a transistor using an oxide semiconductor layer as a channel is attracting attention because of its off-state leakage current characteristics. If the oxide semiconductor layer is used as a channel, however, a high-resistance metal-oxide layer is formed between the oxide semiconductor layer and metal electrodes, which may cause an increase in the contact resistance.